Electronic cassette and operating method thereof

ABSTRACT

An electronic cassette comprises a main battery which is detachably attached to a battery loading unit, and a sub-battery which supplies electricity to a bias power circuit and so on in substitution for the main battery. A power source selector changes the power source to the sub-battery from the main battery when it is judged that a replacement operation of the main battery is started. Since supply of the electricity to the bias power circuit is continued, and the bias voltage continues being applied to a photoelectric converter without a break, there is no need to perform a photoelectric conversion stabilizing process and an offset correction image detecting process after turning on of the main power. Therefore, a start-up time TR 2  is largely shortened from a start-up time TR 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2014-199345, filed Sep. 29, 2014. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic cassette used forradiography and an operation method of an electronic cassette.

2. Description Related to the Prior Art

In medical radiography, e.g., radiography, an electronic cassette isused widely. The electronic cassette has a constitution that an imagedetection unit (it is also called a flat panel detector (FPD)) whichdetects an X-ray image of a subject such as a patient is accommodatedwithin a housing of portable type.

For example, the image detection unit is composed of a scintillatorwhich converts X-rays into visible light and emits it and a photodetection substrate which detects the emitted visible light from thescintillator and converts it into an electric signal. The photodetection substrate includes a plurality of pixels arrangedtwo-dimensionally. The pixel is composed of a photoelectric converterwhich responds to visible light and produces and accumulates electriccharge, and a switching element such as a TFT (Thin Film Transistor)which is connected to the photoelectric converter for reading theelectric charge accumulated in the photoelectric converter. A biasvoltage is applied to the photoelectric converter from a bias powercircuit.

As stated in Japanese Patent Laid-Open Publication No. 2014-160046,there is an electronic cassette usable without a cable, by comprising aradio communication unit performing radio communication with an externalequipment such as a control unit controlling operations of theelectronic cassette and a battery to supply electricity to drive theelectronic cassette. In Japanese Patent Laid-Open Publication No.2014-160046, the battery supplies electricity to sections of theelectronic cassette such as a bias power circuit.

As stated in Japanese Patent Laid-Open Publication No. 2014-142355, abattery is detachably attached in a housing. When remaining capacity ofthe battery has reduced, an operator such as a radiology technicianremoves the battery from the housing and replaces with another batterywhich has been charged.

In addition, Japanese Patent Laid-Open Publication No. 2014-160046states that an offset correction is performed to remove influence of afixed pattern noise due to operating environment of the electroniccassette from an X-ray image. As is generally known, the offsetcorrection is a process that the image detection unit detects an image(image for offset correction) in the state that X-rays are notirradiated, and the image for offset correction is deductedpixel-by-pixel basis from an X-ray image which the image detection unitdetected based on X-rays which transmitted through an object.

When the battery is detached from the housing for exchange, sinceelectricity is not supplied to the bias power circuit and othersections, a bias voltage is not applied to the photoelectric converter.Therefore, it takes a long start-up time from turning on of the mainpower of the electronic cassette after the exchange of the battery to astate that photographing preparation is completed to perform X-raysphotography. The reason is that a detection of an image for offsetcorrection should be performed again when an application of bias voltageis stopped once and then re-started, and it takes a long time forstabilization of the operation of the photoelectric converter which isnecessary to detect the image for offset correction precisely.

So far, when a battery is exchanged in the middle of radiography,re-start of the radiography is delayed due to the long start-up time,and a stress to an object and a reduction in photography efficiency arecaused. Therefore, it has been demanded shortening of the start-up timewhen a battery is exchanged.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electronic cassettewhich can shorten a start-up time when a battery is exchanged, and anoperation method of the electronic cassette.

To achieve the above and other objects, an electronic cassette of thepresent invention comprises an image detection unit, a housing ofportable type, a photoelectric converter, a bias power circuit, a mainbattery, a sub-battery and a power source selector. The image detectionunit has a pixel to detect a radiation image of an object based onradiation that penetrated the object. The housing of portable typecontains the image detection unit. The photoelectric converterconstitutes the pixel, generates electric charge in response to visiblelight converted from the radiation, and accumulates the electric charge.The bias power circuit applies bias voltage to the photoelectricconverter. The main battery is detachably attached to the housing andsupplies electricity to the bias power circuit. The sub-battery supplieselectricity to the bias power circuit in substitution for the mainbattery. The power source selector judges whether a replacementoperation of the main battery is started, and changes the power sourcefor the bias power circuit from the main battery to the sub-battery whenthe power source selector judges that the replacement operation isstarted.

It is preferable that the electronic cassette further comprises adetachment detection unit which detects whether a detachment operationof the main battery from the housing is started, and the power sourceselector judges that the replacement operation is started and changesthe power source of the bias power circuit to the sub-battery when thedetachment detection unit detects a start of the detachment operation.

It is preferable that the electronic cassette further comprises a mainpower switch for operating on/off of a main power of the electroniccassette and a first timer which times a first elapsed time after themain power switch is turned off, and the power source selector continuesto supply of the electricity from the main battery till the firstelapsed time reaches a predetermined first set time after the main powerswitch is turned off, and stops the supply of the electricity from themain battery after the first elapsed time reaches the first set timeeven when the detachment detection unit does not detect the start of thedetachment operation.

It is also preferable that the electronic cassette further comprises amain power switch for operating on/off of a main power of the electroniccassette, and the power source selector judges that the replacementoperation is started when the main power switch is turned off.

It is preferable that the electronic cassette further comprises a secondtimer which times a second elapsed time after the power source selectorchanges the power source to the sub-battery, and the power sourceselector continues to supply of the electricity from the sub-batterytill the second elapsed time reaches a predetermined second set time,and stops the supply of the electricity from the sub-battery after thesecond elapsed time reaches the second set time.

It is preferable that the power source selector changes the power sourcefor the bias power circuit to the main battery when the main power isturned on while the sub-battery supplies electricity.

It is preferable that the pixel is constituted of the photoelectricconverter and a switching element connected to the photoelectricconverter to read electric charge, the electronic cassette furthercomprises a driving circuit which drives the switching element and afirst power supply circuit which supplies a drive voltage for theswitching element to the driving circuit, and electricity is supplied tothe first power supply circuit from the sub-battery.

It is preferable that the electronic cassette further comprises a signalprocessor which converts electric charge into an image signalconstituting the radiation image and a second power supply circuit whichsupplies a drive voltage to the signal processor, and electricity issupplied to the second power supply circuit from the sub-battery.

It is preferable that the electronic cassette further comprises animpact history acquiring unit including an impact detection sensor whichdetects an impact to the housing, a storage section which stores anoutput from the impact detection sensor, and a storage control sectionwhich controls recording of the output from the impact detection sensorto the storage section, and a third power supply circuit which suppliesa drive voltage to the impact history acquiring unit, and electricity issupplied to the third power supply circuit from the sub-battery.

It is preferable that the electronic cassette further comprises acharging circuit which charges the sub-battery with electricity from themain battery.

In an operating method of an electronic cassette of the presentinvention, the electronic cassette comprises an image detection unit, ahousing of portable type, a photoelectric converter, a bias powercircuit, a main battery and a sub-battery. The image detection unit hasa pixel to detect a radiation image of an object based on radiation thatpenetrated the object. The housing of portable type contains the imagedetection unit. The photoelectric converter constitutes the pixel,generates electric charge in response to visible light converted fromthe radiation, and accumulates the electric charge. The bias powercircuit applies bias voltage to the photoelectric converter. The mainbattery is detachably attached to the housing and supplies electricityto the bias power circuit. The sub-battery supplies electricity to thebias power circuit in substitution for the main battery. The operatingmethod of the electronic cassette comprises steps of judging whether areplacement operation of the main battery is started, and changing thepower source for the bias power circuit from the main battery to thesub-battery when it is judged in the judging step that the replacementoperation is started.

According to the present invention, since a sub-battery which supplieselectricity to the bias power circuit in substitution for the mainbattery is provided in addition to a main battery which is detachablyattached to the housing and supplies electricity to the bias powercircuit, and the power source is changed to the sub-battery from themain battery when it is judged that a replacement operation of the mainbattery is started, a bias voltage is continually applied to aphotoelectric converter even during the replacement operation of themain battery. Accordingly, it is not necessary to take time to stabilizean operation of the photoelectric converter and to detect an image foroffset correction again when the replacement operation of the mainbattery is completed and the main power is turned on. Therefore, thestart-up time after the replacement of the battery of the electroniccassette can be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe more apparent from the following detailed description of thepreferred embodiments when read in connection with the accompanieddrawings, wherein like reference numerals designate like orcorresponding parts throughout the several views, and wherein:

FIG. 1 is an external perspective diagram of an electronic cassetteviewed from a front face side;

FIG. 2 is an external perspective diagram of the electronic cassetteviewed from a back face side;

FIG. 3 is a block diagram of an electrical constitution of theelectronic cassette;

FIG. 4 is a schematic diagram of correction sections applying variouscorrections to an X-ray image, such as an offset correction;

FIG. 5 is a schematic diagram of a photographing preparation process;

FIG. 6 is a block diagram of a power feeding unit;

FIG. 7 is a schematic diagram indicating a case of supplying electricityfrom a main battery;

FIG. 8 is a schematic diagram indicating a case of supplying electricityfrom a sub-battery;

FIG. 9 is a flow chart of an operation of the electronic cassette;

FIG. 10 is a timing chart indicating an operation of the electroniccassette in case that the main battery is removed for a purpose exceptan exchange of the main battery;

FIG. 11 is a timing chart indicating an operation of the electroniccassette in case that the main battery is removed for exchanging themain battery;

FIG. 12 is a block diagram of a power feeding unit of a secondembodiment provided with a main power switch;

FIG. 13 is a flow chart of an operation of an electronic cassette of thesecond embodiment;

FIG. 14 is a timing chart indicating an operation of the electroniccassette in case that the main power switch is turned off for a purposeexcept an exchange of the main battery in the second embodiment;

FIG. 15 is a timing chart indicating an operation of the electroniccassette in case that the main power switch is turned off for exchangingthe main battery in the second embodiment;

FIG. 16 is a block diagram of a power feeding unit of a third embodimentprovided with the main power switch;

FIG. 17 is a block diagram of an electrical constitution of anelectronic cassette of a fourth embodiment provided with an impacthistory acquiring unit;

FIG. 18 is a block diagram of the impact history acquiring unit; and

FIG. 19 is a schematic diagram indicating a case of supplyingelectricity from the sub-battery in the fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

In FIG. 1 and FIG. 2, an electronic cassette 10 is used for example formedical radiography, and comprises an image detection unit 15, a circuitboard 16, a radio communication unit 17, a female connector 18, a mainbattery 19, a sub-battery 20 and a housing 21 of portable type whichcontains these parts.

The housing 21 is constituted of a front face 22 where X-ray is incidenton, abase face 23 opposite to the front face and four side faces 24, 25,26, 27, and has a cuboid shape. The housing 21 is formed for example ofconductive resin, and functions as an electromagnetic shield whichprevents invasion of electromagnetic noise to the electronic cassette 10and emission of electromagnetic noise from the electronic cassette 10 tooutside. For example, the housing 21 has the almost same size as that ofa film cassette, an IP (Imaging Plate) cassette or a CR (ComputedRadiography) cassette, and follows the international standard ISO(International Organization for Standardization) 4090:2001.

The electronic cassette 10 is detachably attached in a holder of anupright radiographing table or a decubitus radiographing table to keep aposture that an X-ray source irradiating X-ray and the front face 22 arefacing each other. In addition, the electronic cassette 10 may be usedalone for an object lying on a bed or an object who cannot move withhis/her own strength such as an elderly person or an emergency case.Furthermore, in case the housing 21 has almost the same size as that ofa film cassette, an IP cassette or a CR cassette, the electroniccassette 10 can be attached to existing photographing tables for thesecassettes.

A rectangular opening is formed in the front face 22, and a transmissionplate 28 is attached to the opening. The transmission plate 28 is formedof a carbon material which is lightweight and has high rigidity andradiolucency. In addition, in the housing 21, an indicator (notillustrated) such as an LED (Light Emitting Diode) is provided forindicating operating conditions of the electronic cassette 10 such ason/off of the main power, capacity of the main battery 19 and completionof photographing preparation.

The image detection unit 15 is composed of a scintillator 30 and a photodetection substrate 31. The scintillator 30 and the photo detectionsubstrate 31 are laminated in order of the scintillator 30 and the photodetection substrate 31 from the front face 22 side where X-ray isincident on. The scintillator 30 has a fluorescent substance such asCsI:Tl (Thallium-activated Cesium Iodide) or GOS (Gd₂O₂S:Tb,Terbium-activated Gadolinium Oxysulfide), converts X-ray incidentthrough the transmission plate 28 into visible light and emits it.

The photo detection substrate 31 detects visible light emitted from thescintillator 30 and converts it into an electric signal. The circuitboard 16 controls drive of the photo detection substrate 31 andgenerates an X-ray image based on the electric signal output from thephoto detection substrate 31.

The radio communication units 17 are provided one by one at the cornerwhere the side face 25 intersects the side face 26 and at the cornerwhere the side face 24 intersects the side face 27. The radiocommunication unit 17 is covered with a cover 32 formed of anonconductive material such as a resin having radio wave permeability.The radio communication unit 17 performs radio communication with acontrol unit (not illustrated) which controls operation of theelectronic cassette 10 and transmits variety of information such as anX-ray image. When the radio communication unit 17 is used, theelectronic cassette 10 can be used without a cable, by being driven byelectricity from the main battery 19.

The female connector 18 is provided in the side face 27, to enable awired communication with the control unit. A male connector 33 isconnected to the female connector 18. To the male connector 33, one endof a cable 34 is connected to establish wire a wired communicationbetween the electronic cassette 10 and the control unit. The other endof the cable 34 is connected to a connector (not illustrated) to beconnected to the control unit. The female connector 18 is protected bybeing covered with a lid 35 when the male connector 33 is not connectedto it, for example at the time of using the wireless communicationfunction.

The electronic cassette 10 receives not only variety of information butalso electricity from the control unit through the female connector 18.When the female connector 18 and the male connector 33 are connected,the electronic cassette 10 is driven with electricity from the controlunit.

The main battery 19 is composed of a rechargeable secondary battery. Themain battery 19 supplies electricity to sections of the electroniccassette 12 through a power feeding unit 59 (refer to FIG. 3). In acentral part of the base face 23, a battery loading unit 36 in which themain battery 19 is detachably loaded is provided. FIG. 2 illustrates thestate that the main battery 19 is loaded in the battery loading unit 36.In addition, the battery loading unit 36 is provided with a well-knownfalling prevention/release mechanism (not illustrated) such as a lockmechanism and the unlock mechanism described in Japanese PatentLaid-Open Publication No. 2014-142355, to fix the main battery 19 to thebattery loading unit 36 for preventing falling of the main battery 19and release the fixation.

On/off of the main power of the electronic cassette 10 links toattachment/detachment of the main battery 19 to/from the battery loadingunit 36. Specifically, the main power is turned off when the mainbattery 19 is detached from the battery loading unit 36, and is turnedon when the main battery 19 is attached to the battery loading unit 36.

The sub-battery 20 is incorporated in the housing 21. The sub-battery 20supplies electricity to a bias power circuit 50 (refer to FIG. 3)through the power feeding unit 59 in substitution for the main battery19, when a replacement operation of the main battery 19 is started. Thesub-battery 20 is composed of a rechargeable secondary battery like themain battery 19. The sub-battery 20 may be composed of a rechargeablestorage element such as an electric double layer capacitor or a lithiumion capacitor.

In FIG. 3, pixels 40 arranged into a two-dimensional matrix of N-by-Marray, N scanning lines 41 and M signal lines 42 are provided on a glasssubstrate (not illustrated) in the photo detection substrate 31. Thescanning lines 41 are arranged in Y direction along the column directionof the pixels 40 at a predetermined pitch, and extend in X directionalong the line direction of the pixels 40. The signal lines 42 extendingin the Y direction are arranged in the X direction at a predeterminedpitch. The scanning line 41 is orthogonal to the signal line 42, and thepixel 40 is provided corresponding to a crossing point of the scanningline 41 and the signal line 42. Note that N and M are integers no lessthan 2, for example, N, M≈2000. In addition, the arrangement of thepixels 40 is not limited to the square arrangement of this embodiment,and may be a honeycomb arrangement.

As is generally known, the pixel 40 includes a photoelectric converter44 which generates electric charge (electron-hole pair) by incidence ofvisible light and accumulates the charge and a TFT 45 which is aswitching element. The photoelectric converter 44 has the structure thatan upper electrode and a lower electrode are provided on upper and lowersides of a semiconductor layer producing electric charge. For example,the semiconductor layer is a PIN (p-intrinsic-n) type, an N-type layeris formed on the upper electrode side, and a P-type layer is formed onthe lower electrode side. The TFT 45 has a gate electrode connected tothe scanning line 41, a source electrode connected to the signal line42, and a drain electrode connected to the lower electrode of thephotoelectric converter 44.

A bias line 46 is connected to the upper electrode of the photoelectricconverter 44. The provided bias lines 46 of the number (M) of rows ofthe pixels 40 are connected to a single bus line 47. The bus line 47 isconnected to the bias power circuit 50. Through the bus line 47 and thebias line 46 which is the child line of the bus line 47, a positive biasvoltage is applied to the upper electrode of the photoelectric converter44 from the bias power circuit 50. By the application of the positivebias voltage, an electric field occurs in the semiconductor layer. Thephotoelectric converter 44 is used in a reverse bias state. The electronof the electron-hole pairs which occurred in the semiconductor layer byphotoelectric conversion moves to the upper electrode and is absorbed inthe bias line 46, and the positive hole moves to the lower electrode andis collected as electric charge.

In the circuit board 16, the above-mentioned bias power circuit 50, ascanning circuit 55, a signal processor 56, a memory 57, a power supplycircuit 58, a power feeding unit 59 and a controller 60 for controllingthese sections are provided.

The scanning circuit 55 is connected to the end of the each scanningline 41, and generates a gate pulse G (K) (K=1 to N) to drive the TFT45. The scanning circuit 55 corresponds to a driving circuit, and thegate pulse G (k) corresponds to a drive voltage. The controller 60drives the TFT 45 through the scanning circuit 55 to make the imagedetection unit 15 perform a pixel reset operation, an accumulationoperation and an image reading operation. In the pixel reset operation,a dark electric charge is read from the pixel 40 for resetting(cancelling). In the accumulation operation, electric chargecorresponding to a reached X-ray dose is accumulated in the pixel 40. Inthe image reading operation, the electric charge is read from the pixel40.

In the pixel reset operation and the image reading operation, thescanning circuit 55 applies the gate pulse G (K) sequentially to theeach scanning line 41, to turn on the TFTs 45 connected to the eachscanning line 41 sequentially line by line. In the accumulationoperation, since the gate pulse G (K) is not applied from the scanningcircuit 55, the TFT 45 is in an off state.

The signal processor 56 is connected to the end of the each signal line42. The signal processor 56 has an integral amplifier, a gain amplifier,a CDS (Correlated Double Sampling) circuit, a multiplexer and an A/D(Analog-to-Digital) converter (these are not illustrated).

The integral amplifier accumulates electric charge input from the signalline 42 and integrates it, and outputs an analog voltage value (imagesignal) corresponding to the integrated electric charge. The integralamplifier has an amplifier reset switch. When the amplifier reset switchis turned on, the electric charge accumulated in the integral amplifieris reset (canceled). In the pixel reset operation, the electric chargeis reset without an image signal being output from the integralamplifier.

In the image reading operation, after an image signal corresponding tothe electric charge is output from the integral amplifier, the amplifierreset switch is turned on and the electric charge is reset.

The gain amplifier amplifies the image signal output from the integralamplifier with a predetermined gain level. The CDS circuit applies awell-known correlative double sampling process to the image signalamplified by the gain amplifier, and removes a reset noise component ofthe integral amplifier from the image signal. The integral amplifier,the gain amplifier and the CDS circuit are provided for each of M linesof the signal line 42.

The multiplexer selects the CDS circuit of line 1 to line M sequentiallyline by line, and inputs the image signal output from the each CDScircuit to the A/D converter in a serial manner. The A/D converterapplies an A/D conversion process to an input image signal, and output adigital image signal. The digital image signal output from the A/Dconverter is stored as an X-ray image in the memory 57.

The power supply circuit 58 supplies the gate pulse G (K) to thescanning circuit 55. In addition, the power supply circuit supplies adrive voltage to the signal processor 56. Accordingly, the power supplycircuit 58 corresponds to a first power supply circuit and a secondpower supply circuit. In addition, though illustration is omitted, thepower supply circuit 58 supplies a drive voltage to other electriccircuits such as the radio communication unit 17, the memory 57 and thecontroller 60.

The each battery 19, 20 and a detachment detection unit 61 are connectedto the power feeding unit 59. The power feeding unit 59 supplieselectricity from the main battery 19 or the sub-battery 20 to eachsection of the electronic cassette 10, such as the bias power circuit 50and the power supply circuit 58. The power feeding unit 59 is composedof a DC (Direct Current)-DC converter which converts a DC voltage fromthe each battery 19, 20 into a voltage whose value is adapted to thesection to which the voltage is supplied, a voltage stabilizer whichstabilizes a value of the converted voltage, and so on. In addition,though illustration is omitted, since the female connector 18 isconnected to the power feeding unit 59, when the female connector 18 andthe male connector 33 are connected, the power feeding unit 59 receiveselectricity from the control unit through the female connector 18 andsupplies the electricity from the control unit to each section.

The detachment detection unit 61 detects whether a detachment operationto detach the main battery 19 from the battery loading unit 36 isstarted. For example, the detachment detection unit 61 is composed of amicro switch or a photo sensor to detect whether the cancellationoperation is applied to the falling prevention/release mechanism of thebattery loading unit 36 by an operator. When the cancellation operationis applied to the falling prevention/release mechanism, the detachmentdetection unit 61 outputs a detection signal indicating the start of thedetachment operation to the power feeding unit 59.

The controller 60 outputs the X-ray image stored in the memory 57 to theradio communication unit 17 or the female connector 18. In addition, thecontroller 60 receives variety of information from the control unitinput through the radio communication unit 17 or the female connector18, and controls each section depending on the received information. Forexample, the controller 60 receives a photography condition of theradiography, to change a gain level of the gain amplifier of the signalprocessor 56 depending on the photographing condition and control astart timing of the image reading operation depending on an X-rayirradiation time included in the photographing condition.

In FIG. 4, correction sections 65, 66, 67 are provided in the controller60, to apply various correction processing such as an offset correction,a sensitivity correction and a defective pixel correction to the X-rayimage stored in the memory 57. The each correction section 65-67accesses the memory 57 to read the X-ray image, applies variouscorrection processing to the X-ray image, then writes the processedX-ray image back to the memory 57.

The offset correction section 65 deducts an image for offset correction,which is detected by the controller 60 making the image detection unit15 perform the image reading operation while X-ray is not irradiated,from the X-ray image pixel by pixel basis. Accordingly, fixed patternnoise due to an individual difference of the signal processor 56 and ause environment of the electronic cassette 10 is removed from the X-rayimage.

The sensitivity correction section 66 corrects an unevenness of thesensitivity of the photoelectric converter 44 of the each pixel 40, anunevenness of output characteristics of the signal processor 56 and soon based on sensitivity correction data. The defective pixel correctionsection 67 performs linear interpolation of a pixel level of a defectpixel with a pixel level of neighboring normal pixels 40, based ondefect pixel information having an abnormal pixel level generated at thetime of shipment or a periodic inspection. The X-ray image to which suchthe correction processes are applied are transmitted to the control unitthrough the radio communication unit 17 and the female connector 18. Inaddition, the image for offset correction, the sensitivity correctiondata and the defect pixel information are stored in an internal memoryof the controller 60 which is not illustrated, and are read to the eachcorrection section 65-67 at the time of the each correction process atappropriate timing.

In FIG. 5, the controller 60 makes the image detection unit 15 perform aphotographing preparation process when the bias power circuit 50 startsapplication of the bias voltage to the photoelectric converter 44, forexample when the main battery 19 is attached to the battery loading unit36 and the main power of the electronic cassette 10 is turned on at thetime of start-up of a medical facility. The photographing preparationprocess is for making the electronic cassette 10 be in the state thatthe photographing preparation is completed and the radiography ispossible. The photographing preparation process mainly includes astabilization process of the operation of the photoelectric converter 44(photoelectric conversion stabilizing process) and a process to make theimage detection unit 15 detect an image for offset correction (offsetcorrection image detecting process).

The photoelectric conversion stabilizing process is performed for thepurpose of detecting an accurate image for offset correction, because animage for offset correction which reflects an use condition of theelectronic cassette 10 accurately may not be obtained when the image foroffset correction is detected before operation of the photoelectricconverter 44 becomes stabilized after the main power is turned on andthe application of bias voltage to the photoelectric converter 44 fromthe bias power circuit 50 is re-started. Specifically, in thephotoelectric conversion stabilizing process, the image detection unit15 is not operated and the offset correction image detecting process isprohibited until operation of the photoelectric converter 44 isstabilized. For example, the photoelectric conversion stabilizingprocess takes a time more than half of the time required for whole ofthe photographing preparation process.

The offset correction image detecting process is performed after thephotoelectric conversion stabilizing process. For example, when the mainpower is turned on at the time of start-up of a medical facility, theoffset correction image detecting process is performed because a usecondition of the electronic cassette 10 might have changed since thetime of former detection of the image for offset correction. In theoffset correction image detecting process, the image reading operationis performed after a dark electric charge is reset in the pixel resetoperation, to prevent that a dark electric charge component issuperimposed on the image for offset correction.

In addition, the photographing preparation process further includes aprocess to activate the radio communication unit 17 and establish awireless communication with the control unit, an initialization processof the scanning circuit 55, the signal processor 56, the memory 57 andso on. These processes are carried out parallel to the photoelectricconversion stabilizing process.

As illustrated in FIG. 6, the power feeding unit 59 comprises a powersource selector 70, a timer 71 and a charging circuit 72. A detectionsignal is input into power the source selector 70 from the detachmentdetection unit 61. The power source selector 70 changes a power sourcefor the bias power circuit 50, the power supply circuit 58 and so ondepending on the detection signal, by switching between the each battery19, 20.

The timer 71 clocks a second elapsed time TP2 (refer to FIG. 10 and FIG.11) after the power source selector 70 changes the power source to thesub-battery 20. Accordingly, the timer 71 corresponds to a second timer.When the second elapsed time TP2 reached a predetermined second set timeTS2 (refer to FIG. 10 and FIG. 11), the timer 71 outputs a notificationsignal indicating this state to the power source selector 70. The secondset time TS2 is an enough time from the start to the end of areplacement operation of the main battery 19 (e.g., five minutes).

The charging circuit 72 receives the electricity from the main battery19 and charges the sub-battery 20. While the main battery 19 is attachedto the battery loading unit 36, the charging circuit 72 operates. Whensub-battery 20 was in a condition of the full charge, charging circuit72 stops charge of sub-battery 20. In addition, though illustration isomitted, the charging circuit 72 has a function to receive theelectricity from the control unit through the female connector 18 andcharge the main battery 19.

As illustrated in FIG. 7, when the main battery 19 is attached to thebattery loading unit 36 and the main power is turned on, the powersource selector 70 selects the main battery 19 as the power source andmake the main battery 19 supply electricity to the bias power circuit50, the power supply circuit 58 and so on.

In addition, the power source selector 70 judges whether a replacementoperation of the main battery 19 is started. As illustrated in FIG. 8,when the power source selector 70 judges that the detachment operationof the main battery 19 is started (the replacement operation isstarted), the power source selector 70 changes the power source from themain battery 19 to the sub-battery 20 and make the sub-battery 20 supplyelectricity to the bias power circuit 50, the power supply circuit 58and so on.

When the main battery 19 runs short and is to be exchanged, a releaseoperation of the falling prevention/release mechanism of the batteryloading unit 36 is executed to detach the main battery 19 from thebattery loading unit 36. Then the removed main battery 19 is chargedwith an exclusive battery charger (not illustrated), and another mainbattery 19 which has been charged is loaded in the battery loading unit36. In other words, the replacement operation of the main battery 19 isstarted from the release operation of the falling prevention/releasemechanism of the battery loading unit 36.

Accordingly, in this embodiment, the power source selector 70 judgesthat the replacement operation of main battery 19 is started when therelease operation of the falling prevention/release mechanism of thebattery loading unit 36 is executed, then changes the power source tothe sub-battery 20 as illustrated in FIG. 8. Therefore, for example atan ending time of the medical facility when the electronic cassette 10has not been activated for certain amount of time (at least longer thanthe second set time TS2) and the main battery 19 is removed from thebattery loading unit 36, in other words, even in case that the mainbattery 19 is removed for an intention except the exchange of the mainbattery 19, the power source is changed to the sub-battery 20 by thepower source selector 70 as illustrated in FIG. 8.

The power source selector 70 continues to supply of the electricity fromsub-battery 20 as illustrated in FIG. 8 till the second elapsed time TP2reaches the second set time TS2 (TP2<TS2). And when the second elapsedtime TP2 reaches the second set time TS2 (TP2=TS2) and the notificationsignal indicating this state is input by the timer 71, the power sourceselector 70 stops the supply of the electricity from the sub-battery 20.In addition, when the main battery 19 is attached to the battery loadingunit 36 and the main power is turned on while the sub-battery 20supplies the electricity (before the second elapsed time TP2 reaches thesecond set time TS2), the power source selector 70 changes a powersource to the main battery 19 as illustrated in FIG. 7.

Next, an operation of the above-described embodiment is explained withreference to a flowchart of FIG. 9 and timing charts of FIG. 10 and FIG.11. Note that the explanation starts from the state that the mainbattery 19 is already attached and the main power is turned on, and allsections are activated by the electricity supplied from the main battery19.

In step S100 of FIG. 9 to FIG. 11, the power source selector 70 monitorsthe detection signal from the detachment detection unit 61, and judgeswhether the replacement operation (detachment operation) of the mainbattery 19 is started. While the replacement operation is not started,power supply from the main battery 19 to each section is continued (Noin step S100).

When an operator operates the falling prevention/release mechanism ofbattery loading unit 36 for the release operation, the detection signalis input into the power source selector 70 from the detachment detectionunit 61. The power source selector 70 judges that the replacementoperation of the main battery 19 is started, upon receiving thedetection signal (YES in step S100). Then the power source selector 70changes the power source from the main battery 19 to the sub-battery 20.In addition, at the same timing, the clock of the second elapsed timeTP2 is started with the timer 71 (step S110). In this way, electricityis supplied to the bias power circuit 50, the power supply circuit 58and so on from the sub-battery 20 (step S120).

When the attachment of the main battery 19 to the battery loading unit36 and the turning on of the main power are not completed before thesecond elapsed time TP2 reaches the second set time TS2 (No in stepsS130 and S140), the supply of the electricity from the sub-battery 20 iscontinued until the second elapsed time TP2 reaches the second set timeTS2. Note that when electricity is supplied from the sub-battery 20 andthe sub-battery 20 runs short, the sub-battery 20 is charged withelectricity from the main battery 19 through the charging circuit 72.

When the second elapsed time TP2 reaches the second set time TS2 (YES instep S130), the notification signal indicating this state is output fromthe timer 71 to the power source selector 70. Then the supply of theelectricity from the sub-battery 20 is stopped by the power sourceselector 70 (step S150). Therefore, as for the cases that the attachmentof the main battery 19 and the turning on of the main power are notcompleted before the second elapsed time TP2 reaches the second set timeTS2, for example that the main battery 19 is removed for an intentionexcept the exchange of the main battery 19, the application of the biasvoltage to the photoelectric converter 44 is stopped because theelectricity to the bias power circuit 50 is no longer supplied fromeither of the batteries 19 and 20.

When the main battery 19 is loaded and the main power is turned on inthe state that the application of the bias voltage to the photoelectricconverter 44 is stopped (YES in step S160), the power source is changedto the main battery 19 from the sub-battery 20 by the power sourceselector 70 (step S170), and electricity is supplied to the bias powercircuit 50, the power supply circuit 58 and so on from the main battery19 (step S180). In this case, since the application of the bias voltageto the photoelectric converter 44 had been stopped, the photoelectricconversion stabilizing process and the offset correction image detectingprocess are performed by the controller 60 (steps S190 and S200).

When the attachment of the main battery 19 and the turning on of themain power are completed before the second elapsed time TP2 reaches thesecond set time TS2 (NO in step S130, YES in step S140), the powersource is changed to the main battery 19 from the sub-battery 20 by thepower source selector 70 (step S210), and electricity is supplied to thebias power circuit 50, the power supply circuit 58 and so on from themain battery 19 (step S220), as same as steps S170 and S180.

In this case also, as well as the case that the second elapsed time TP2reaches the second set time TS2 (YES in step S130), and the main battery19 is loaded and the main power is turned on (YES in step S160), thepower source is sequentially changed from the main battery 19 to thesub-battery 20 when the main battery 19 is detached, and from thesub-battery 20 to the main battery 19 when the main battery 19 isattached. However, the supply of the electricity to the bias powercircuit 50 is continued by the each battery 19, 20, and the bias voltagecontinues being applied to the photoelectric converter 44 without abreak. Accordingly, in this case the photoelectric conversionstabilizing process and the offset correction image detecting processare not performed. Therefore, a start-up time TR2 of this caseillustrated in FIG. 11 is largely shortened from a start-up time TR1illustrated in FIG. 10 in which the photoelectric conversion stabilizingprocess and the offset correction image detecting process are performed.

Accordingly, even when the main battery 19 is changed in the middle ofradiography, the radiography can be re-started relatively in a shorttime, without waiting for a long time. Therefore, photography efficiencycan be improved and a stress to an object can be reduced.

Even in case the main battery 19 is removed for an intention except theexchange of the main battery 19, when the attachment of the main battery19 and the turning on of the main power are completed before the secondelapsed time TP2 reaches the second set time TS2, the photoelectricconversion stabilizing process and the offset correction image detectingprocess are not performed and the start-up time becomes TR2. Therefore,there is the secondary effect to be able to restart radiography in ashort time even when detachment/attachment of the main battery 19 iscarried out for saving electric power.

Since the sub-battery 20 is used for power supply alternatively onlyduring the replacement operation of the main battery 19, it is enoughthat the sub-battery 20 has a capacity to supply electricity at leastduring a period from a start to a completion of the replacementoperation of the main battery 19. Therefore, the sub-battery 20 can bevery small in capacity and size in comparison with the main battery 19.

Since the main battery 19 needs to be removed for exchanging the mainbattery 19, an overlook of a start timing of the replacement operationof the main battery 19 can be prevented by detecting a start of thedetachment operation of the main battery 19 as the start of thereplacement operation of the main battery 19.

In case the power source is changed to the sub-battery 20 after the mainbattery 19 is removed from the battery loading unit 36 and the supply ofthe electricity from the main battery 19 is completely cut off, aninstantaneous interruption of power supply occurs. To prevent theinstantaneous interruption of power supply, in this embodiment, a startof the detachment operation of the main battery 19 is detected to changethe power source to the sub-battery 20 before the supply of theelectricity from the main battery 19 is completely cut off.

In addition, since the timer 71 clocks the second elapsed time TP2 afterthe power source is changed to the sub-battery 20 and the supply of theelectricity from sub-battery 20 is stopped when the second elapsed timeTP2 reaches the second set time TS2, a battery power waste can beprevented.

Note that the timer 71 can be omitted. In case the timer 71 is notprovided, the sub-battery 20 can play a role as the timer 71, by usingthe sub-battery 20 having a capacity which makes a discharging time fromthe full charge state to the zero state agree with the second set timeTS2, and the charging circuit 72 charging the sub-battery 20 only whenthe capacity of the sub-battery 20 becomes zero.

Second Embodiment

In the first embodiment mentioned above, attachment and detachment ofthe main battery 19 to and from the battery loading unit 36 link on/offof the main power of the electronic cassette 10. However, a main powerswitch 80 may be provided in the housing 21 for operating on/off of themain power.

In this case, as illustrated in FIG. 12, an operation signal of the mainpower switch 80 is input into a power source selector 81, in addition tothe detection signal of the detachment detection unit 61. For exchangingthe main battery 19, at first the main power switch 80 is turned off,the main battery 19 is removed from the battery loading unit 36thereafter.

In addition, in this embodiment, a timer 82 times a first elapsed timeTP1 (refer to FIG. 14 and FIG. 15) after the main power switch 80 isturned off, in addition to the second elapsed time TP2 of the firstembodiment mentioned above. In other words, the timer 82 corresponds tothe first timer and the second timer. When the first elapsed time TP1reached a predetermined first set time TS1 (refer to FIG. 14 and FIG.15), the timer 82 outputs a notification signal indicating this state tothe power source selector 81. Note that the first set time TS1 is nomore than the second set time TS2 of the first embodiment.

When the main power switch 80 is turned on, the power source selector 81selects the main battery 19 as the power source and makes the mainbattery 19 supply electricity to the bias power circuit 50, the powersupply circuit 58 and so on. After the main power switch 80 is turnedoff, the power source selector 81 continues to supply of the electricityfrom the main battery 19 until the first elapsed time TP1 reaches thefirst set time TS1. In addition, the power source selector 81 stops thesupply of the electricity from the main battery 19 after the firstelapsed time TP1 reaches the first set time TS1, even when the detectionsignal is not input from the detachment detection unit 61.

Furthermore, when the detection signal is input from the detachmentdetection unit 61 before the first elapsed time TP1 reaches the firstset time TS1, the power source selector 81 judges that the replacementoperation of the main battery 19 is started, and change the power sourceto the sub-battery 20 from the main battery 19.

Next, an operation of this embodiment is explained with reference to aflow chart of FIG. 13 and timing charts of FIG. 14 and FIG. 15. Notethat the explanation starts from the state that the main power switch 80is turned on, and all sections are activated by the electricity suppliedfrom the main battery 19.

In step S300 of FIG. 13 to FIG. 15, the power source selector 81 judgeswhether the main power switch 80 is turned off. In the first embodimentmentioned above, when the detachment operation of the main battery 19 isstarted, the power source is changed to the sub-battery 20 from the mainbattery 19 by the power source selector 70. However, in this embodiment,even when the main power switch 80 is turned off (YES in step S300),supply of the electricity from the main battery 19 is continued by thepower source selector 81. In addition, timing of the first elapsed timeTP1 is started concurrently by the timer 81. Accordingly, electricity issupplied continually from the main battery 19 to the bias power circuit50, the power supply circuit 58 and so on (step S310).

When the detection signal is not input from the detachment detectionunit 61 before the first elapsed time TP1 reaches the first set timeTS1, in other words, when the detachment operation of the main battery19 is not started (NO in steps S320 and S330), the supply of theelectricity from the main battery 19 is continued until the firstelapsed time TP1 reaches the first set time TS1.

When the first elapsed time TP1 reaches the first set time TS1 (YES instep S320), a notification signal indicating this state is output fromthe timer 82 to the power source selector 81. Then the supply of theelectricity from the main battery 19 is stopped by the power sourceselector 81 (step S340). Therefore, as for the cases that the detectionsignal is not input from the detachment detection unit 61 before thefirst elapsed time TP1 reaches the first set time TS1, for example thatthe main power switch 80 is turned off for an intention except theexchange of the main battery 19, the application of the bias voltage tothe photoelectric converter 44 is stopped because the electricity to thebias power circuit 50 is no longer supplied from either of the batteries19 and 20. About the subsequent processes, description is omittedbecause it is the basically same to step S160 and subsequent processesof the first embodiment mentioned above, though a trigger of turning themain power on is changed to the turning on of the main power switch 80from the attachment of the main battery 19.

When the detection signal is input from the detachment detection unit 61before the first elapsed time TP1 reaches the first set time TS1, inother words, when the detachment operation of the main battery 19 isstarted (NO in step S320 and YES in step S330), the processing istransferred to step S110 of the first embodiment mentioned above. Inother words, the power source selector 81 changes the power source fromthe main battery 19 to the sub-battery 20, and makes the sub-battery 20supply electricity to the bias power circuit 50, the power supplycircuit 58 and so on. About the subsequent processes, description isomitted because it is the basically same to step S110 and subsequentprocesses of the first embodiment mentioned above, though a trigger ofturning the main power on is changed to the turning on of the main powerswitch 80 from the attachment of the main battery 19.

Also in this embodiment, the supply of the electricity to the bias powercircuit 50 is continued by the each battery 19, 20, and the bias voltagecontinues being applied to the photoelectric converter 44 without abreak. Accordingly, the photoelectric conversion stabilizing process andthe offset correction image detecting process are not needed. Therefore,a start-up time TR2 illustrated in FIG. 15 is largely shortened from thestart-up time TR1 illustrated in FIG. 14.

In addition, when the main power switch 80 is turned off for anintention except the exchange of the main battery 19 as illustrated inFIG. 14, the sub-battery 20 does not operate. Therefore, since usingfrequency of the sub-battery 20 can be reduced in comparison with thefirst embodiment in which the sub-battery 20 operates even when the mainpower switch 80 is turned off for an intention except the exchange ofthe main battery 19, degradation of the sub-battery 20 can besuppressed. Unlike the main battery 19 which can be replaced with a newone immediately after being deteriorated, the sub-battery 20 is providedin the housing 21 thus it cannot be replaced or repaired easily.Therefore, it is important that a frequency of troublesome maintenancesuch as exchange or repair of the sub-battery 20 is reduced bysuppressing the degradation of the sub-battery 20.

Third Embodiment

In the embodiments mentioned above, upon detecting a start of thedetachment operation of the main battery 19, it is judged that thereplacement operation of the main battery 19 is started. However, incase the main power switch 80 is provided like in the second embodimentmentioned above, it may be judged that the replacement operation of themain battery 19 is started when the main power switch 80 is turned off.

In this case, as illustrated in FIG. 16, only an operation signal of themain power switch 80 is input into the power source selector 90. Thedetachment detection unit 61 is not provided. In addition, the timer 71is used like the first embodiment mentioned above. The power sourceselector 90 judges that the replacement operation of the main battery 19is started when the main power switch 80 is turned off, and changes thepower source to the sub-battery 20. Since there is no need to providethe detection unit 61, production cost can be reduced.

Regarding an operation of this embodiment, description is omittedbecause it is the basically same with the first embodiment, though atrigger of starting the replacement operation of the main battery 19 ischanged to the turning off of the main power switch 80 from thedetection of the start of the detachment operation of the main battery19, and a trigger of turning the main power on is changed to the turningon of the main power switch 80 from the attachment of the main battery19.

In case the main power switch 80 is provided in the housing 21 like inthe second and third embodiments mentioned above, since it is necessaryto form an opening in the housing 21 where the main power switch 80 isattached into, waterproofness of the housing 21 decreases. Accordingly,it is more preferable that attachment and detachment of the main battery19 to and from the battery loading unit 36 link on/off of the main powerof the electronic cassette 10 as in the first embodiment, than the casethat the main power switch 80 is provided in the housing 21 like in thesecond and third embodiments.

Note that the main power switch may be an operating member provided inthe housing 21 like the main power switch 80 of the second and thirdembodiment, or may be a software operating by on/off command from thecontrol unit without providing an exclusive operating member.

In the embodiments mentioned above, the sub-battery 20 supplieselectricity also to the power supply circuit 58 which supplies a drivevoltage to neighboring electric circuits such as the scanning circuit55, the signal processor 56, the memory 57 and the controller 60.However, only the bias power circuit 50 needs electricity from thesub-battery 20 to omit the photoelectric conversion stabilizing processand the offset correction image detecting process, the sub-battery 20may supply electricity only to the bias power circuit 50.

In case the sub-battery 20 supplies electricity to the power supplycircuit 58, though the start-up time TR2 can be more shorten, thesub-battery 20 needs high capacity which requires upsizing of thesub-battery 20. By supplying electricity only to the bias power circuit50, the sub-battery 20 can be smaller with a smaller capacity.

Fourth Embodiment

In case the electronic cassette 10 is carried from a photography roomwhere a decubitus radiographing table or an upright radiographing tableis installed to a sickroom or so on where there is an object for beingused alone, the electronic cassette 10 may hit on the floor or the wallby mistake. Therefore, as the electronic cassette 10, there is onehaving a function to detect an impact to the housing 21 with a sensor.When the detected impact is abnormal, histories such as the detectedtime and the sensor output at that time may be stored in a storagesection, and a warning that the electronic cassette 10 may have brokendown may be displayed.

A fall or a collision of the electronic cassette 10 may occur not onlyduring the transportation of electronic cassette 10 but also during thereplacement operation of the main battery 19. However, the main power isturned off during the replacement operation of the main battery 19. Incase that electricity is not supplied to the sensor to detect an impact,the storage section to store impact histories and so on, an impact thatoccurred during the replacement operation of the main battery 19 is notrecorded as a history. Therefore, in this embodiment, electricity issupplied from the sub-battery 20 to the power supply circuit whichsupplies a drive voltage to the function to acquire a history of animpact to the housing 21.

As illustrated in FIG. 17, an impact history acquiring unit 95 isprovided in the electronic cassette of this embodiment. A power supplycircuit 96 supplies a drive voltage to the impact history acquiring unit95 in addition to the neighboring electric circuits such as the scanningcircuit 55, the signal processor 56, the memory 57 and the controller60, like the power supply circuit 58 of the first embodiment mentionedabove. In other words, the power supply circuit 96 corresponds to thethird power supply circuit.

As illustrated in FIG. 18, the impact history acquiring unit 95 has animpact detection sensor 100, a storage control section 101 and a storagesection 102. The impact detection sensor 100 detects an impact to thehousing 21. For example, the impact detection sensor 100 is a three-axesacceleration sensor, which detects an acceleration in each of three-axesdirections that are a depth wise direction of the housing 21 at theright angle to the front face 22 and the base face 23, a directionparallel to the side faces 24 and 25, and a direction parallel to theside faces 26 and 27. By an output from the impact detection sensor 100,a direction and a degree of an impact can be found.

The storage control section 101 controls recording of an output from theimpact detection sensor 100 the to the storage section 102. When anoutput from the impact detection sensor 100 is more than a predeterminedthreshold, the storage control section 101 makes the storage section 102store histories such as the detected time and the output from the impactdetection sensor 100 at that time.

The storage control section 101 outputs a history stored in the storagesection 102 in to the controller 60. The controller 60 displays awarning that the electronic cassette 10 may have broken down through anindicator. Note that, the history may be sent to the control unitthrough the radio communication unit 17 or the female connector 18, sothat the warning that the electronic cassette 10 may have broken downwill be displayed on a display of the control unit.

As illustrated in FIG. 19, when the power source selector 70 judges thatthe replacement operation of the main battery is started, the powersource selector 70 changes the power source from the main battery 19 tothe sub-battery 20 and make the sub-battery 20 supply electricity to thebias power circuit 50, the power supply circuit 96 and so on.Accordingly, the impact history acquiring unit 95 operates even duringthe replacement operation of the main battery 19. Therefore, an impactthat occurred during the replacement operation of the main battery 19can be recorded as a history.

A single power supply circuit which is common to the scanning circuit55, the signal processor 56, other neighboring electric circuits, theimpact history acquiring unit 95 and so on may be used like the powersupply circuit 58 of the first embodiment and the power supply circuit96 of the fourth embodiment. Alternatively, it may be provided powersupply circuits separately for the parts such as the scanning circuitand the signal processor.

In the above embodiments, the main power is turned off when the mainbattery 19 is detached from the battery loading unit 36 or when the mainpower switch 80 is operated. However, the main power may be constitutedto be turned off automatically when there is no operation during apredetermined time in the state that the main power is turned on.

Note that, the constitution of the image detection unit 15 is notlimited to the constitution that the scintillator 30 is arranged at theX-ray incidence side of the photo detection substrate 31 like the firstembodiment, and it may be a constitution that the scintillator 30 isarranged at the side opposed to the X-ray incidence side of the photodetection substrate 31. In this case, the scintillator 30 absorbs X-raythat penetrated the photo detection substrate 31 and generates visiblelight, and the photo detection substrate 31 detects the visible light.In addition, though the TFT-type image detection unit is illustrated inthe above embodiments, a CMOS (Complementary Metal OxideSemiconductor)-type image detection unit may be used. Furthermore, thepresent invention can be applied to not only the electronic cassette tophotograph based on X-rays, but also an electronic cassette tophotograph based on other radiations such as gamma rays.

Various changes and modifications are possible in the present inventionand may be understood to be within the present invention.

What is claimed is:
 1. An electronic cassette comprising: an imagedetection unit having a pixel to detect a radiation image of an objectbased on radiation that penetrated the object; a housing of portabletype which contains the image detection unit; a photoelectric converterwhich constitutes the pixel, generates electric charge in response tovisible light converted from the radiation, and accumulates the electriccharge; a bias power circuit which applies bias voltage to thephotoelectric converter; a main battery which is detachably attached tothe housing and supplies electricity to the bias power circuit; asub-battery which supplies electricity to the bias power circuit insubstitution for the main battery; and a power source selector whichjudges whether a replacement operation of the main battery is started,and changes the power source for the bias power circuit from the mainbattery to the sub-battery when the power source selector judges thatthe replacement operation is started.
 2. The electronic cassetteaccording to claim 1, further comprising a detachment detection unitwhich detects whether a detachment operation of the main battery fromthe housing is started, wherein the power source selector judges thatthe replacement operation is started and changes the power source of thebias power circuit to the sub-battery when the detachment detection unitdetects a start of the detachment operation.
 3. The electronic cassetteaccording to claim 2, further comprising: a main power switch foroperating on/off of a main power of the electronic cassette; and a firsttimer which times a first elapsed time after the main power switch isturned off, wherein the power source selector continues to supply of theelectricity from the main battery till the first elapsed time reaches apredetermined first set time after the main power switch is turned off,and stops the supply of the electricity from the main battery after thefirst elapsed time reaches the first set time even when the detachmentdetection unit does not detect the start of the detachment operation. 4.The electronic cassette according to claim 1, further comprising a mainpower switch for operating on/off of a main power of the electroniccassette, wherein the power source selector judges that the replacementoperation is started when the main power switch is turned off.
 5. Theelectronic cassette according to claim 1, further comprising a secondtimer which times a second elapsed time after the power source selectorchanges the power source to the sub-battery, wherein the power sourceselector continues to supply of the electricity from the sub-batterytill the second elapsed time reaches a predetermined second set time,and stops the supply of the electricity from the sub-battery after thesecond elapsed time reaches the second set time.
 6. The electroniccassette according to claim 1, wherein the power source selector changesthe power source for the bias power circuit to the main battery when themain power is turned on while the sub-battery supplies electricity. 7.The electronic cassette according to claim 1, wherein the pixel isconstituted of the photoelectric converter and a switching elementconnected to the photoelectric converter to read electric charge,wherein the electronic cassette further comprises a driving circuitwhich drives the switching element and a first power supply circuitwhich supplies a drive voltage for the switching element to the drivingcircuit, and wherein electricity is supplied to the first power supplycircuit from the sub-battery.
 8. The electronic cassette according toclaim 1, further comprising: a signal processor which converts electriccharge into an image signal constituting the radiation image; and asecond power supply circuit which supplies a drive voltage to the signalprocessor, wherein electricity is supplied to the second power supplycircuit from the sub-battery.
 9. The electronic cassette according toclaim 1, further comprising: an impact history acquiring unit includingan impact detection sensor which detects an impact to the housing, astorage section which stores an output from the impact detection sensor,and a storage control section which controls recording of the outputfrom the impact detection sensor to the storage section; and a thirdpower supply circuit which supplies a drive voltage to the impacthistory acquiring unit, wherein electricity is supplied to the thirdpower supply circuit from the sub-battery.
 10. The electronic cassetteaccording to claim 1, further comprising a charging circuit whichcharges the sub-battery with electricity from the main battery.
 11. Anoperating method of an electronic cassette, the electronic cassettecomprising: an image detection unit having a pixel to detect a radiationimage of an object based on radiation that penetrated the object; ahousing of portable type which contains the image detection unit; aphotoelectric converter which constitutes the pixel, generates electriccharge in response to visible light converted from the radiation, andaccumulates the electric charge; a bias power circuit which applies biasvoltage to the photoelectric converter; a main battery which isdetachably attached to the housing and supplies electricity to the biaspower circuit; and a sub-battery which supplies electricity to the biaspower circuit in substitution for the main battery, the operating methodof the electronic cassette comprising steps of: judging whether areplacement operation of the main battery is started; and changing thepower source for the bias power circuit from the main battery to thesub-battery when it is judged in the judging step that the replacementoperation is started.